[GRLUG] RAID levels

[Mike Williams]

Do you think it's possible to construct a table of number of drives 
available versus recommended RAID level? Not that it would be an 
absolute bible, but a place to start.

I'm thinking
2 drives -> RAID 1 (obviously)
3 drives -> RAID 1 + spare
4 drives -> RAID 5 + spare? RAID 6? RAID 10?
5 drives -> RAID 6
6+ drives -> RAID 6 + spare

Fairly often, like in the 5-drive box that started this, the question is 
whether a drive that will not add any storage space is better as a hot 
spare or as a live, already functioning member of the array. In the case 
of 3 drives, if you're really worried about the array going down, would 
it be better to run a 3-way mirror rather than a 2-way one with a hot 
spare? The only thing you gain with the hot spare option is a vastly 
different amount of wear on the spare since it isn't spinning until it's 
needed. What's more likely: the 2nd half of a mirror failing before you 
get the data copied or 2 of 3 drives failing before you buy and install 
a replacement? I know statistics as well as the next guy, but these 
calculations get arcane fast!

On 10/02/2011 09:29 PM, Richard Nienhuis wrote:
> To bring this back to its actual application...  Its a personal music 
> server made from scrap parts.  It doesn't require 5 9's uptime of 
> hands free operation.  The most likely failure mode of this thing is 
> that one day a drive will fail on reboot/startup.  Hot spare isn't 
> going to save it from a second drive failure at startup.  For its 
> application integrity is a priority over everything else.  Downtime is 
> a relatively inconsequential cost.
>
> Hot spare is good if you have an extra one available.  In this case 
> the 5 drives are of better use in RAID 6 rather than just having one 
> sitting around.  If he gets another drive at some point then its 
> painless to add it as a hot spare.  Plus I don't think saving a few 
> minutes is worth the extra hassle since if a drive does fail he is 
> probably going to be right there.  That array will have a rebuild time 
> of 7 hours or so.  Not a big time savings.
>
> On Sun, Oct 2, 2011 at 8:14 PM, Adam Tauno Williams 
> <awilliam at whitemice.org <mailto:awilliam at whitemice.org>> wrote:
>
>     On Fri, 2011-09-30 at 18:55 -0400, Richard Nienhuis wrote:
>     > Performance issues are going to be inconsequential for a machine
>     > playing  music.  In degraded mode there is probably still enough
>     > performance.  Also hot spare isn't going to spare you from the
>     time to
>     > rebuild the array.
>
>     ????   I just can't express in words how strongly, completely,
>     absolutely, and utterly I disagree with the statement: "Hot spare
>     isn't
>     going to spare you from the time to rebuild the array."
>
>     That is like saying ice is hot and water is dry.
>
>     A hot spare means - Rebuilding the array actually starts!!!!
>
>     The sooner it starts the sooner it is complete and the redundancy
>     underlying the data is restored.  A RAID solution without a hot
>     spare is
>     a sports car with only three tires.
>
>
>
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